In the related art, there are known fluorescence observation apparatuses of the type that acquires a white-light image representing the shape of biological tissue and a fluorescence image representing fluorescence from a lesion and that displays a fluorescence region in the fluorescence image as superimposed on the white-light image (e.g., see Patent Literature 1 and Patent Literature 2). With such an apparatus, since the lesion is displayed as the fluorescence region on the white-light image, it is possible to readily ascertain the positional relationship between the tissue and the lesion.
Specifically, according to Patent Literature 1, the gradation values of a fluorescence image are assigned to one of a red component image, a green component image, and a blue component image constituting a color white-light image, whereby the fluorescence region is displayed as a red, green, or blue region on the white-light image. In this case, in the region where the fluorescence region is superimposed, the white-light image contains information about the original gradation values, i.e., information about the shape of the tissue, which provides the advantage that it is possible to observe the shape of the tissue. According to Patent Literature 2, the fluorescence region in the white-light image is painted using pseudo-colors. In this case, the visibility of the fluorescence region is improved.